Cube corner cavity based retroreflectors with transparent fill material

ABSTRACT

Retroreflective sheeting includes a body layer having a structured surface with recessed faces forming cube corner cavities. A reflective film is disposed at least on the recessed faces, and a fill material fills the cube corner cavities. The fill material comprises radiation-curable materials, adhesives, or both, and preferably transparent radiation-curable pressure-sensitive adhesives. The fill material preferably forms a continuous layer covering both the recessed faces and upper portions of the structured surface. A transparent cover layer preferably contacts the fill material layer.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This patent application is a division of application Ser. No.09/228,367, filed Jan. 11, 1999. The present invention relates to U.S.application Ser. No. 09/227,963, “Cube Comer Cavity BasedRetroreflectors and Methods For Making Same”, filed on Jan. 11, 1999 andincorporated by reference.

BACKGROUND

[0002] The present invention relates generally to retroreflectivearticles such as sheeting. More particularly, the invention relates tosuch articles or sheetings in which retroreflective elements comprisereflective faces arranged to form a cavity.

[0003] The reader is directed to the glossary at the end of thespecification for guidance on the meaning of certain terms used herein.

[0004] Cube corner retroreflective sheetings can generally becategorized as those that use a rear-surface body layer and those thatuse a front-surface body layer. Commercially available cube cornerretroreflective sheetings are of the former type, in which a thintransparent body layer has a substantially planar front surface and arear structured surface comprising a plurality of geometric structuresof pyramidal shape, some or all of which include three reflective facesconfigured as a cube corner element. Light is incident on the planarfront surface, passes through the thickness of the body layer, and isretroreflected by the cube corner elements back through the frontsurface. In some known embodiments, a reflective coating such asaluminum is applied to the rear structured surface, followed by anadhesive layer that covers and conforms to some extent to the shape ofthe structured surface. However, in general no reflective coating isrequired so long as a clean air interface can be maintained at thestructured surface, in which case reflections occur by total internalreflection.

[0005] Some known cube corner retroreflective sheeting constructions usea front-surface body layer, in which the body layer has a frontstructured surface. See, e.g., U.S. Pat. No. 3,712,706 (Stamm), U.S.Pat. No. 4,127,693 (Lemelson), and U.S. Pat. No. 4,656,072 (Coburn, Jr.et al.), and PCT Publication WO 89/06811 (Johnson et al.). The frontstructured surface comprises a plurality of reflective faces arranged toform cube corner cavities. For this reason such retroreflective sheetingis referred to herein as cube corner cavity based retroreflectivesheeting. A thin metal film can be applied to the structured surface toenhance reflectivity of the faces. Incident light does not penetratethrough the body layer but rather is reflected by the faces forming thecube corner cavities. In some embodiments a cover layer that doestransmit incident light is provided on top of the structured surface toprotect the cavities from dirt or other degradation, with portions ofthe cover layer extending into and filling in the cube corner cavitiesof the structured surface. In other embodiments a cover layer is sealedor adhered to the structured surface by a colored pressure- orheat-sensitive adhesive that cancels, removes, or obliteratesretroreflectivity of the structured surface.

[0006] One advantage of cube corner cavity-based retroreflectivesheeting is the ability to use a much wider variety of materialcompositions for the body layer than is otherwise possible, since itneed not be optically clear. Another advantage is the ability to formcertain types of structured surfaces in the body layer more rapidly thanit takes to form a negative copy of such structured surfaces inrear-surface body layer constructions. This is because molds used toform the structured surface of a front-surface body layer can havegrooves that are essentially unbounded in the direction of the groove.In contrast, molds used to form the structured surface of a rear-surfacebody layer typically have an array of closed (cube corner) cavitiesbounded by a plurality of inverted grooves, i.e., ridges. The unboundedgrooves of the former molds are easier to fill with body layer materialthan the array of closed cavities provided on the latter molds.

[0007] Unfortunately, this latter advantage can be essentially nullifiedin constructions where the cube corner cavities in the body layer arefilled with a transparent substance. Filling the cavities with such asubstance, referred to as a fill material, is desirable to increase theentrance angularity of the sheeting by refracting highly off-axisincident light closer to the symmetry axis of the cube corner element,as well as to keep dirt or other debris out of the cavities. But suchfilling is undesirable insofar as it requires forcing material into anarray of closed cavities. Such filling is also undesirable to the extentit exposes the body layer to excessive heat, mechanical stress, or otherprocess conditions that could compromise the fidelity of the structuredsurface.

[0008] Constructions of the type described would benefit from fillmaterials having properties that make them easy to fill into the cubecorner cavities of the body layer, preferably with minimal risk ofdamaging the fidelity of the structured surface. Preferred fillmaterials should be compatible with relatively low cost, highflexibility, and high visibility sheeting constructions.

BRIEF SUMMARY

[0009] Certain radiation-curable materials, particularlyradiation-curable pressure-sensitive adhesives, have been found toexhibit significant manufacturing and/or construction advantages whenused as fill materials for cube corner cavity based retroreflectivesheeting.

[0010] Retroreflective articles are disclosed having a body layer with astructured surface in which recessed faces define cube corner cavities.A transparent adhesive material fills the cube corner cavities. Theadhesive material is preferably a pressure-sensitive adhesive. In oneembodiment, a release liner covers the fill material. In anotherembodiment, a transparent cover layer takes the place of the releaseliner. The cover layer adds durability to the article, and can alsoincorporate dyes, colorants, or the like to affect the appearance of thesheeting or to convey information.

[0011] Methods are disclosed in which a film of reflective material isapplied at least to recessed faces of a body layer structured surface,such recessed faces forming cube corner cavities. A flowable compositionsuch as a resin is applied to the structured surface. The composition isone suitable for forming a transparent PSA, or one that is radiationcurable and suitable for bonding to the film of reflective material, or,preferably, both. After the composition has substantially completelyfilled the cube corner cavities, the composition is crosslinked orotherwise cured by exposure to radiation such as UV light. After theexposure step, the crosslinked composition bonds to the reflective filmand preferably also to a transparent cover layer.

[0012] To reduce cost while maintaining functionality and durability,the constructions preferably utilize thermoplastic materials for thebody layer and the cover layer. Good flexibility of sheeting articlescan be aided by the use of fill materials whose elastic modulus aftercrosslinking is less than about 50,000 psi (345×10⁶ Pascals), andpreferably less than about 25,000 psi (172 MPa).

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a perspective view of a retroreflector where a top coverlayer thereof is shown only partially laminated to a body layer toreveal cube corner cavities formed in the body layer;

[0014]FIG. 2 is a cross-sectional view of a portion of theretroreflector of FIG. 1 taken along line 2-2, and additionally showinga fill material filling the cube corner cavities and bonding the coverlayer to the body layer;

[0015]FIG. 3 depicts a process for fabricating cube corner cavity-basedretroreflective sheeting; and

[0016] FIGS. 4A-C demonstrate a self-replicating phenomenon observedwith some types of fill materials.

[0017] In the drawings, the same reference symbol is used forconvenience to indicate elements that are the same or that perform thesame or a similar function.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0018] In FIG. 1, a portion of a retroreflective sheeting 10 is shownenlarged. Sheeting 10 comprises a body layer 12 having a structuredsurface 14, and a transparent cover layer 16. Structured surface 14includes recessed faces 18 and top surfaces 20, the recessed faces 18forming cube corner cavities 22. The recessed faces 18 are shown shadedfor visual effect. In a preferred construction, a vapor-coated film ofreflective material such as aluminum, silver, or the like is exposed onthe recessed faces but masked on top surfaces 20, whether by the absenceof such reflective material or the presence of a masking material on thetop surfaces. Alternatively, the film of reflective material can beexposed on both faces 18 and surfaces 20, but the surfaces 20 arephysically roughened to impart a diffuse reflectivity to the film. Instill another alternative, top surfaces 20 can be eliminated by allowingthe recessed faces to converge or intersect along sharp edges.

[0019]FIG. 2 shows a sectional view of a portion of the sheeting 10,additionally showing a discontinuous film 24 of reflective material onthe recessed faces 18, and a fill material 26 that fills cube cornercavities 22. Fill material 26 is preferably sufficiently transparent toallow light rays to propagate through it with minimal degradation ofretroreflective efficiency. In contrast to known constructions, fillmaterial 26 forms a strong bond not only with transparent cover layer 16but also with film 24 and with any exposed portions of body layer 12.Thus, fill material 26 is preferably coextensive with the structuredsurface 14 and, for ease of construction, forms a substantiallycontinuous layer that covers both the recessed faces and the topsurfaces of the structured surface. In an alternative embodiment thefill material can be coextensive with structured surface 14 butdiscontinuous, encapsulated by a network of bonds directly between coverlayer 16 and top surfaces 20. This may be advantageous where, forexample, a direct bond between the cover layer and the body layer can bemade stronger than one in which the fill material and/or the reflectivematerial are interposed. However, embodiments having a continuous fillmaterial layer are preferred in part because the construction process ismore robust by avoiding the stringent requirement of having to apply aprecise amount of fill material to the structured surface-just enough tosubstantially fill the cavities, but not so much that the fill materialcovers the upper portions of the structured surface in a way thatinterferes with the network of bonds between the cover layer and upperportions of the body layer. The continuous fill material layerembodiments also allow the fill material to flow from one cube cornercavity to another before the fill material is solidified bycross-linking. Finally, in constructions where the fill materialfunctions as a bonding agent between the body layer and the cover layer,a continuous fill material layer improves bond strength by increasingthe surface area of the bond.

[0020] Market demands often require sheetings of various colors fordifferent applications, or sheetings that have symbols or other indicia.These distinctive visual effects can be realized by adding colorants,dyes, or the like to cover layer 16 as is known. Manufacturing,inventory, and storage costs become a consideration when a variety ofdifferent sheetings each having different cover layers 16 must all bemanufactured and stored in sufficient quantity so that each type will beavailable upon receipt of an order. It has been found that cube cornercavity-based sheeting that uses a PSA as the fill material hassurprising versatility that can be used to reduce these costs. Inparticular, the body layer, reflective film, and the fill material canall be prepared, but then instead of applying the transparent coverlayer 16, a standard release liner is laminated to the fill material.The release liner need only protect the fill material (the PSA) fromcontamination during storage until a particular type of sheeting iscalled for. At that time, the release liner is stripped and theappropriate cover layer is applied to the sheeting in a simplelamination process.

[0021] Heat-activated adhesives can be used as the fill material withsimilar beneficial results. Examples of such adhesives are Nucrel brandethylene acid copolymer resins, sold by E. I. du Pont de Nemours andCompany. An advantage of heat-activated adhesives is that the releaseliner can in some cases be eliminated from the construction. Theintermediate sheeting, without the cover layer and without a releaseliner, can be stored in a roll under standard storage conditions withoutadhering to itself. A disadvantage is that the sheeting must be heatedto activate the adhesive properties when the cover layer is applied.

[0022] A PSA or heat-activated adhesive can also be used as the fillmaterial in applications where no cover layer is required. For example,it may be desirable to apply the front side of the sheeting to atransparent substrate such as window glass in a vehicle or a building.The sheeting thus retroreflects light incident from the opposite side ofthe window glass. For such applications a sheeting with a transparentPSA fill material and a release liner is particularly well suited.

[0023] For most other applications, sheeting 10 preferably includesanother thin adhesive layer 28 on the back side of body layer 12 so thesheeting can be applied to a substrate of interest. Where layer 28 is aPSA, another release liner 28 a is also included. Layer 28 need not betransparent and thus it can comprise a wider variety of PSAs than layer26. However, if the same composition is used for layer 26 and 28,manufacturing inventory can be reduced.

[0024] Radiation-curable materials that are not PSAs or PSA precursorscan also be used to advantage. See, e.g., Examples 1-4 below. Suchmaterials should have sufficient clarity to promote goodretroreflectivity, have relatively low viscosity during application tothe structured surface, and also have a sufficiently low shrinkage sothat it maintains intimate contact with the structured surface aftercuring.

[0025] Transparent PSA fill materials, particularly radiation curablefill materials disclosed herein, tend to be relatively expensivecompared to fill materials disclosed in the prior art. Therefore, tokeep production costs down it is advantageous when using the disclosedfill materials to use relatively inexpensive thermoplastic materials forbody layer and cover layer compositions. However, other materials suchas radiation-curable materials are also contemplated.

[0026] Product flexibility is often desirable in sheeting applications.At the same time, the sheeting is expected to have a robust constructioncapable of withstanding various types of physical abuse. Theseconflicting requirements can be satisfied to some extent in the presentconstructions by the use of a fill material layer that has a relativelylow elastic modulus, less than about 50,000 psi (345 MPa), andpreferably less than about 25,000 psi (172 MPa), to provide flexibility.The fill material layer is sandwiched between and protected by the bodylayer and cover layer.

[0027]FIG. 3 (not drawn to scale) depicts a process for making cubecorner cavity-based sheeting with the preferred fill materials. Notshown in the figure, body layer 12 described above is provided thestructured surface 14 by embossing, or by other processes used to makeconventional rear-surface body layers. Also not shown, a film ofreflective material 24 is then applied either discontinuously as shownin FIG. 2, or continuously on both the recessed faces and the topsurfaces 20. It is not necessary that the structured surface 14 have topsurfaces 20, although such surfaces are useful for controlling daytimeappearance and, in some instances, for improved bonding. In the absenceof top surfaces 20, recessed faces 18 of adjacent cube corner elementsintersect to form sharp edges.

[0028] Film 24 can comprise metals such as aluminum, silver, nickel,tin, copper, or gold, or combinations thereof, or can comprisenon-metals such as a multilayer dielectric stack. Such films can beapplied by known physical or chemical deposition techniques, such asvacuum evaporation, sputtering, chemical vapor deposition (“CVD”) orplasma-enhanced CVD, electroless deposition, and the like, dependingupon the type of film desired. A given film can include multiple layers,including layers that promote adhesion to the body layer, barrierlayers, and protective overcoat layers. A suitable film forpolycarbonate-based body layers comprises about 1 nm thick titaniumdioxide layer formed by sputtering titanium onto the body layer,followed by a 100 nm thick layer of evaporated aluminum. The titaniumdioxide layer acts both as an adhesion promoter and a barrier layer tocounteract pinholes typically present in the aluminum vapor coat.

[0029] The body layer 12 so prepared is then sent through a fillmaterial application station 30. Generally, the more easily the fillmaterial fills the cavities, the faster (and hence cheaper) the processcan be run. Preferred fill materials have properties that permit rapidfilling of the cube corner cavities. The fill material should adhere tothe reflective film-covered recessed cube corner cavity faces withoutdamaging the reflective film or other parts of the structured surface.

[0030] At station 30, a flowable fill material composition 32 is appliedto the structured surface 14 ahead of a knife coater 34 whose positionrelative to a base plate 36 is adjusted to form a layer of composition32 on the structured surface. If desired, vacuum assistance or inert gaspurging can be used at the point of filling to further facilitate theoperation. Some compositions 32, discussed in more detail below, have arelatively low viscosity at station 30 to permit rapid filling of theclosed cube corner cavities. In contrast to prior art thermoplastic fillmaterials such compositions can exhibit these low viscosities atrelatively low process temperatures, well below the glass transitiontemperature of typical body layer materials. For some fill materials,process temperatures at or around ambient room temperature areachievable.

[0031] Composition 32 is suitable for forming a fill material that bondswell to all other parts of the sheeting that it contacts, including thereflective film 24, exposed portions of the body layer 12, and any coverlayer (with the exception of a release liner that may be used as atemporary cover layer).

[0032] As an aside, where a discontinuous reflective film 24 is used,some combinations of fill material and body layer material can be usedto produce a covalent bond therebetween for added robustness anddurability. The covalent bond between the fill material and body layercan be formed during exposure to radiation. For example, for a fillmaterial composition comprised of 25 wt. % tetrahydofurfuralacrylate(THF acrylate), 50 wt. % Ebecryl 8402 (available from Radcure), and 25wt. % neopentylglycoldiacrlylate, a suitable body layer can comprise anethylenepropylenedienemonomer (EPDM) based material, such asacrylic-EPDM-styrene (AES) polymers sold under the tradename Centrex(available from Bayer), Luran (available from BASF), or other polymersthat crosslink upon exposure to radiation. Such fill material reactswith these body layer materials at the top surfaces 20 upon exposure tocrosslinking radiation to produce covalent bonds along the top surfaces.

[0033] Turning again to FIG. 3, after composition 32 is applied to thestructured surface at fill material application station 30, the filledbody layer is conveyed to a cover layer laminating station 38. For somefill materials, however, it may be desirable to apply radiation, such aswith a source of radiation 40, after station 30 but before station 38.For example, some compositions 32 are composed of highly monomerizedsyrups which can chemically attack certain reflective films 24 ormigrate through pinholes in the reflective film to attack the underlyingbody layer material. In such cases, it is preferable to polymerize andcrosslink the compositions in situ shortly after application to the bodylayer so that damage to the sheeting can be minimized. However, forother compositions it is desirable that the composition 32 remainflowable at least up to the laminating station 38.

[0034] At station 38, filled body layer 12 is conveyed between pressurerollers 42 a,42 b rotating oppositely as shown. A cover layer 44,unwound from a roll 46, passes through the nip between the rollers andis laminated to body layer 12. The fill material, though preferablystill not fully cross-linked, exhibits sufficient adhesion to hold coverlayer 44 in place. The stippled appearance of composition 32 in FIGS. 3and 4A-C indicates that it is not fully crosslinked and exhibits coldflow.

[0035] In an alternative embodiment, the composition 32 can be appliedto the structured surface 14 by first being applied to the underside ofthe cover layer 44. Then, filling of the cube corner cavities andlamination of the cover layer can take place simultaneously at thelamination station 38.

[0036] Cover layer 44 can comprise a transparent cover layer such aslayer 16, to be used in the final sheeting product, or it can comprise atemporary layer such as a release liner. In either event another sourceof radiation 48 can be used to crosslink composition 32 to increase itsshear and adhesive strength. Such crosslinked fill material is labeled32 a and depicted as cross-hatched rather than stippled. The fillmaterial 32 a does not exhibit significant cold-flow. In a simpleconstruction method, cover layer 44 is a transparent cover layer such aslayer 16. Source 48 is sufficiently intense, and layer 44 has a lowenough absorption for at least some UV wavelengths, or for e-beamradiation, so that crosslinking can be effected through the cover layeras shown. The crosslinked composition 32 a aggressively bonds toreflective film 24, body layer 12, and cover layer 44. In an alternativeconstruction method, the cover layer 44 has a higher UV absorption tobetter protect the remainder of the sheeting from degradation due tosunlight. Body layer 12 and reflective film 24 are then composed ofmaterials having a lower absorption at the relevant UV wavelengths, andsource 48 is disposed underneath instead of above the sheeting so as toexpose the composition 32 to crosslinking radiation through body layer12 and film 24. Silver used as reflective film 24 can be madesufficiently thin to allow adequate transmission in a UV spectral bandlocated at about 360 nm. Alternatively, multilayer dielectric films canbe easily tailored to have a high specular reflectance at the designwavelength and a transmission band in the UV.

[0037] In still another method, cover layer 44 is a release liner. Therelease liner is of conventional design, for example silicone-coatedpaper. The release liner may or may not be transparent at the designwavelength for the sheeting, and it may or may not have a low absorptionof UV radiation. If absorption in the UV is low enough, source 48 cancrosslink composition 32 through layer 44 as shown in FIG. 3. Otherwise,the release liner can be removed just prior to crosslinking, or source48 can be positioned to crosslink composition 32 from below the sheetingas discussed above.

[0038] Sources 40, 48 can be adapted to emit UV light or other forms ofradiation capable of carrying out the prescribed functions, for example,infrared radiation or electron beam radiation.

[0039] Still other methods contemplated herein delay application ofcross-linking radiation from source 48 to take advantage of the coldflow characteristics of composition 32. For example, a temporarysheeting comprising body layer 12, cover layer 44, and fill material 32can be manufactured as shown in FIG. 3 except that source 48 iseliminated and the temporary sheeting is wound up in a roll and placedin storage. During storage, imperfections in the fill material layertend to disappear due to flow of the uncured composition 32 from forcessuch as surface tension or other forces incident to the storageenvironment. After a sufficient time has elapsed, the temporary sheetingcan then be further processed by rapidly passing it near source 48 toexpose composition 32 to radiation sufficient to produce crosslinkedcomposition 32 a, preferably a PSA. In some cases a relatively shortdelay may suffice, so that the temporary sheeting need not be wound upand stored.

[0040] Overall processing speed for the sheeting can be enhanced byusing the delayed curing procedure just described. This is because thefilling operation at station 30 can be accelerated beyond the speed atwhich substantially complete filling of cavities 22 is assured. Becausethe fill material is radiation curable, it remains in its uncuredflowable state substantially indefinitely—whether for seconds, minutes,hours, or days—until crosslinking by exposure to radiation is needed.The radiation-curable materials can thus be used to improve the processof making cube corner cavity-based retroreflective sheeting, making theprocess faster and more robust since the degree of care needed to ensurecomplete filling of the cube corner cavities is not required.

[0041] FIGS. 4A-C depict the sheeting 10 at different times afterapplying fill material to the structured surface 14 and after laminatingcover layer 44 thereto, but before crosslinking the fill material. Theproduction line speed of the body layer 12 has been increased to thepoint where complete filling of the cavities has not occurred, leaving avoid 50 at the apex of each cavity. The cold flow properties of thecomposition 32 advantageously allow the fill material to advance intothe cavities without the application of external forces and, typically,at standard process or storage temperatures, typically about 10 to about40 degrees C. This behavior is referred to as self replication. Ascomposition 32 advances into the cavity, voids 50 shrink (FIG. 4B),eventually disappearing altogether (FIG. 4C). Self replication viashrinkage of the voids is presumed to occur as gas in the voids diffusesinto the fill material. The rate at which self replication occurs isdependent on the type of composition 32 used, the cover layer 44properties (if a cover layer is used), the size of the cube cornercavities and the initial size of gaps 50, and on environmental factorssuch as temperature.

Illustrative Fill Materials

[0042] Suitable fill materials include viscoelastic polymers which canattain the requisite transparency. Preferred materials exhibit cold flowat room temperature which allows the fill material to flow into thecavities of the body layer and also allows any entrapped gas to diffuseout, thus maintaining the optical performance of the sheeting. It isdesirable further that the fill material exhibit little or no shrinkageupon curing such that it maintains intimate contact with the reflectiverecessed faces of the structured surface.

[0043] A preferred class of materials includes acrylic polymers whichmay be pressure-sensitive adhesives at room temperature, orheat-activated adhesives which are substantially non-tacky at roomtemperature but become tacky at higher temperatures. The preferredacrylic polymers and copolymers are formed from acrylic or methacrylicacid esters of non-tertiary alkyl alcohols. The acrylic and methacrylicesters typically have a glass transition temperature below about 0° C.Examples of such monomers include n-butyl acrylate, isooctyl acrylate,2-ethylhexyl acrylate, isononyl acrylate, isodecyl acrylate, decylacrylate, lauryl acrylate, hexyl acrylate, and octadecyl acrylate, orcombinations thereof and the like. Such monomeric acrylic or methacrylicesters are known and many are commercially available.

[0044] The acrylic polymers typically include a copolymerizable monomerhaving a glass transition temperature above 0° C. to enhance shearstrength. Suitable copolymerizable monomers include acrylic acid,methacrylic acid, itaconic acid, n-vinyl pyrrolidine, n-vinylcaprolactam, substituted acrylamides such as N,N dimethylacrylamide,N-vinyl-2-pyrrolidone, N-vinyl caprolactam, acrylonitrile, isobornylacrylate, tetrahydrofurfuryl acrylate, glycidyl acrylate,2-phenoxyethylacrylate, benzylacrylate, acrylonitrile andmethacrylonitrile or combinations thereof.

[0045] Typically, the amounts of acrylate monomer to copolymerizablemonomer can vary from 100 to about 30 parts of acrylate andcorrespondingly, 0 to 70 parts of copolymerizable monomer. The specificamounts of monomers are selected for the desired end use properties.

[0046] The acrylic polymers may be prepared by emulsion polymerization,bulk polymerization, solvent polymerization, and the like, usingappropriate polymerization initiators. Suitable pressure-sensitiveadhesives for the invention are described in, for example, U.S. PatentNo. 5,637,646 (Ellis), U.S. Pat. No. 4,181,752 (Martens et al.), and Re.24,906 (Ulrich), all incorporated herein by reference.

[0047] Various other materials may be added to tailor thecharacteristics of the polymer for the end use. Such materials includecolorants, fluorescent dyes or pigments, chain transfer agents,plasticizers, tackifiers, antioxidants, stabilizers, cross-linkingagents, and solvent.

[0048] The fill material is preferably crosslinked to provide highershear strength. In order to maintain optical performance, it ispreferred that any entrapped gas or voids be allowed to escape orcollapse before the fill material is crosslinked. Suitable cross-linkingagents include those that are free radically copolymerizable with theacrylate monomers, and may be activated by radiation such as ultravioletlight. Additionally, crosslinking may be effected in the absence ofcross-linking agents by means such as electron beam.

[0049] When the fill material is applied to the sheeting insubstantially polymeric form, e.g. as a hot melt coating, entrapped gasis allowed to diffuse, presumably into the fill material, beforecrosslinking. Examples of crosslinking agents suitable for thisapplication include free-radically copolymerizable crosslinking agentssuch as, for example, 4-acryloxybenzophenone,para-acryloxyethoxybenophenone, andpara-N-(methacryloxyethyl)-carbamoylethoxybenophenone. Copolymerizablechemical cross linking agents are typically included in the amount ofabout 0% to about 2%, and preferred in the amount of about 0.025% toabout 0.5%, based on the total weight of monomer(s). Other usefulcopolymerizable crosslinking agents are described in U.S. Pat. No.4,737,559 (Kellen et al.). The crosslinking is effected by ultravioletlight.

[0050] Alternatively, a fill material composition may be polymerized insitu in the cavities of the sheeting by coating a monomeric oroligomeric composition onto the sheeting and polymerizing thecompositions with heat or radiation. In this case, the composition has aviscosity that is sufficiently low prior to polymerization that any gassuch as air diffuse out of the composition quickly prior, and thecomposition flows rapidly and easily to fill the cavities of thesheeting. Suitable crosslinking agents include those mentioned above aswell as materials that crosslink during the polymerization process.Examples of this type of crosslinking agent include multi-functionalacrylates such as 1,6 hexanedioldiacrylate andtrimethylolpropanetriacrylate and substituted triazines described inU.S. Pat. No. 4,330,590 (Vesley et al.) and U.S. Pat. No. 4,329,384(Vesley at al.). These crosslinking agents may be used in amounts offrom about 0.0001% to about 0.005% based on the weight of the monomers.

[0051] The fill material may be applied to the sheeting by any suitablemethod. For example, the polymers can be dispersed in a solvent or anemulsion, coated onto the sheeting, and drying off the solvent or waterto leave the polymer in the cavities of the sheeting. The polymers maybe hot melt coated onto the sheeting using known equipment such asextrusion coaters, rotary rod die coaters, and the like. The polymersmay also be formed in the cavities of the sheeting as described above.Solvent-free processes are preferred because they eliminateenvironmental concerns associated with solvents and avoid formation ofbubbles which may occur during drying of a solvent-containingcomposition.

Body Layer Materials and Cover Layer Materials

[0052] The body layer for retroreflective sheeting as described hereincan be manufactured as a unitary material, e.g. by embossing a preformedsheet with an array of cube corner elements as described above or bycasting a fluid material into a mold. Alternatively, the body layer canbe manufactured as a layered product by casting a layer defining thestructured surface against a preformed flat film analogous to theteachings of PCT Publication No. WO 95/11464 (Benson, Jr. et al.) andU.S. Pat. No. 3,684,348 (Rowland), or by laminating a preformed film toa preformed layer having cube corner cavities. Useful body layermaterials are those that are dimensionally stable, durable, weatherable,and readily formable into the desired configuration. Examples includeacrylics such as Plexiglas brand resin from Rohm and Haas, thermosetacrylates and epoxy acrylates, preferably radiation cured;polycarbonates; polyolefins; polyethylene-based ionomers (marketed underthe name ‘SURLYN’); polyesters; cellulose acetate butyrates; andpolystyrenes. Generally any material that is formable, typically underheat and pressure, can be used. The sheeting can also include colorants,dyes, UV absorbers, or other additives as desired.

[0053] Suitable transparent cover layer materials can also be of singleor multilayer construction and can comprise materials that are opticallytransparent at least at a design wavelength and that are durable andweatherable. Thermoplastic or thermoset polymers, or combinationsthereof, are generally acceptable. Acrylics, vinyl chloride, urethanes,ethylene acrylic acid (EAA) copolymers, polyesters, and fluoropolymersincluding polyvinylidene fluoride are preferred for weatherability.Corrosion inhibitors, UV stabilizers (absorbers), colorants includingfluorescent dyes and pigments, abrasion resistant fillers, solventresistant fillers, and the like can be included to provide desiredoptical or mechanical properties. The cover layer can have graphics,symbols, or other indicia so that the sheeting formed by the combinationof the body layer and cover layer conveys useful information.

[0054] As discussed above, many thermoplastic polymers such as EAA,polyvinyl chloride, polystyrene, polyethylene-based ionomers,polymethylmethacrylate, polyester, and polycarbonate are as a wholerelatively inexpensive and for that reason, to help offset the higherexpense of typical radiation cured fill materials disclosed herein, theyare desirable for use in the body layer and cover layer. The body layeralso preferably has an elastic modulus greater than that of the fillmaterial in order to maintain cube stability during deformation. Thebody layer elastic modulus is preferably greater than about 100,000 psi(690 MPa), measured in accordance with ASTM D882-97 “Standard TestMethod for Tensile Properties of Thin Plastic Sheeting”.

EXAMPLES 1-4

[0055] Four body layers were embossed with a mold to impart a structuredsurface similar to that shown in FIG. 1. The mold had a structuredsurface consisting of three sets of flat-bottomed grooves, and was thenegative replica of a prior mold whose upper portions had been grounddown flat with an abrasive. The embossed body layers were made ofpolycarbonate. The body layers for Examples 1 and 2 had a thickness ofabout 43 mils (1.1 mm) and included sufficient TiO2 filler to make themopaque with a diffuse white surface appearance. Those for Examples 3 and4 had a thickness of about 18 mils (0.46 mm) and included instead a reddye to give a diffuse red surface appearance.

[0056] The structured surface of each body layer consisted essentiallyof three intersecting sets of parallel ridges. Two of the sets, referredto as “secondary” ridge sets, had uniform ridge spacings of about 16mils (408 μm) and intersected each other at an included angle of about70 degrees. The other set of parallel ridges, referred to as the“primary” ridge set, had a uniform spacing of about 14 mils (356 μm) andintersected each of the secondary ridge sets at an included angle ofabout 55 degrees. This produced cube corner cavity matched pairs cantedat an angle of about 9.18 degrees. All of the ridges had substantiallyflat top surfaces whose transverse dimension was about 3.5 mils (89 μm)for the primary grooves and about 2.2 mils (56 μm) for the secondarygrooves. The top surfaces were all non-smooth as a result of theabrasive action on the original mold discussed above, transferred to thebody layers via the replication steps.

[0057] The cube corner elements had a cube depth below the top surfacesof about 5.17 mils (131 μm). A silver film was vacuum deposited onto thestructured surface of each sample to a thickness sufficient to renderthe film opaque yet highly reflective. For Examples 2 and 4, the portionof the silver film disposed on the top surfaces was removed by lightlysanding with an abrasive. The silver film for Examples 1 and 3 was leftundisturbed and continuous.

[0058] A radiation-curable composition was prepared by combining (byweight) 74% Ebecryl 270 (a urethane acrylate available from Radcure),25% Photomer 4127 (propoxylated neopently glycol diacrylate availablefrom Henkel), and 1% Daracure 1173 (a photoinitiator available fromCiba-Geigy). This composition was then flow coated on the structuredsurface of all samples at room temperature to a thickness sufficient tofill the cube corner cavities and cover the top surfaces. Thecomposition was flowable and had a viscosity of about 2000 centipoise (2Pa-s) during filling. The samples were degassed at room temperature in asmall vacuum chamber. Next, when no bubbles remained in the composition,the samples were removed from the chamber and covered with a 7 mil (178μm) thick sheet of photo-grade PET sheeting to eliminate oxygen duringsubsequent curing. A heavy quartz plate having good transparency in theUV was placed on the PET sheeting and curing was then performed throughthe quartz plate and PET sheeting with UV light from a mercury lamp forabout two minutes. The fill material composition had a sufficiently lowshrinkage so that it hardened and bonded to the vapor-coated body layer.The composition did not bond to the PET sheeting, which was thenremoved. The cured composition was substantially clear and smooth butnot permanently tacky. The sheetings so constructed all exhibitedretroreflectivity. The coefficient of retroreflection was measured at a−4 degree entrance angle, 0 degree orientation angle, and at both 0.2and 0.5 degree observation angles, and have not been adjusted to takeinto account the proportion of the structured surface actually occupiedby the cube corner elements: Retro. Coeff. Silver film on (cd/lx/m²)Sample No. Body layer color top surfaces @ 0.2° @ 0.5° 1 White Yes 58 262 White No 46 22 3 Red Yes 28 14.6 4 Red No 22 17

[0059] These measurements demonstrate that the silver film imparts ahigh specular reflectivity to the recessed faces. Samples 2 and 4, withthe silver film exposed selectively on the recessed faces, exhibitednoticeable daytime color (white or red) as a result of the exposed bodylayer at the top surfaces.

EXAMPLES 5-16

[0060] Twelve body layers made of polystyrene (type 498 Styron brand,available from Dow Chemical Co., Midland, Mich.) were embossed with amold to impart a structured surface consisting essentially of threeintersecting sets of parallel ridges. Six of the body layers (Examples5-10) were natural, clear polystyrene and the remaining six (Examples11-16) used polystyrene compounded with a titanium dioxide concentrateto impart diffuse whiteness. The body layers were each about 9 mils (230μm) thick. Two of the three sets of parallel ridges, referred to as“secondary” ridge sets, had uniform ridge spacings of about 5.74 mils(146 μm) and intersected each other at an included angle of about 70degrees. The other set of parallel ridges, referred to as the “primary”ridge set, had a uniform ridge spacing of about 5 mils (127 μm) andintersected each of the secondary ridge sets at an included angle ofabout 55 degrees. This produced cube corner cavity matched pairs havinga cavity depth of about 2.5 mils (64 μm) and an angle of cant of about9.18 degrees. The ridges in each of the three ridge sets did not havetop surfaces as defined herein but rather terminated at sharp upperportions whose transverse dimension was less than 0.0001 inches (2.5μm). A continuous aluminum film about 100 nm thick was vacuum depositedonto the entire structured surface.

[0061] A transparent hot melt PSA composition was prepared according tothe procedure of Example 1 of U.S. Pat. No. 5,753,768 (Ellis) except asindicated below. A 200-gallon (757 liter) stainless steel batch reactorwas charged with: 441.3 kilograms of isooctyl acrylate (“IOA”); 54.4 kgof acrylic acid (“AA”); 0.0017 parts of Vazo™ 52(2,2′-azobis(2,4-dimethylpentanenitrile)) per 100 parts of IOA and AA(“pph”); 0.0084 pph isooctylthioglycoate; 0.5 pph of a 25 weight %solids mixture of 4-acryloxy benzophenone in IOA; and 0.1 pph ofIrganox™ 1010 thermal stabilizer/antioxidant(tetrakis(methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate))methane),manufactured by Ciba-Geigy Corporation. The composition was reacted inthe first adiabatic reaction cycle with the starting reactiontemperature of about 60 degrees C. It reached a solids content of about50% from the reaction.

[0062] After cooling the composition to about 55 degrees C, a mixture of0.004 pph of Vazo™ 52, 0.004 pph of Vazo™ 88(2,2′-azobis(cyclohexanecarbonitrile)), 0.0004 pph of di-t-butylperoxide, 0.004 pph of t-butylperbenzoate, and 0.04125 pphisooctylthioglycoate, 0.5 pph of a 25 weight % solids mixture of4-acryloxy benzophenone in IOA, and 4.54 kilograms of IOA was mixed intothe reaction mixture.

[0063] The composition was then heated to 60 degrees C, held untilpolymerization began, and then reacted adiabatically. After the secondadiabatic reaction cycle was completed, the resulting polymercomposition had a 93% solids content.

[0064] The batch of adhesive thus produced was then vacuum stripped toprovide a pressure-sensitive adhesive polymer having an inherentviscosity of 0.44 dl/g and 0.1% or less residuals. The inherentviscosity was measured by conventional methods using a Cannon-Fenske #50viscometer in a water bath controlled at 25° C., using the flow time of10 ml of a polymer solution (0.2 g per deciliter polymer in ethylacetate). The test procedure followed and the apparatus used aredescribed in Textbook of Polymer Science, F. W. Billmeyer,Wiley-Interscience (Second Edition, 1971), pp. 84-85.

[0065] The prepared PSA composition was substantially completelypolymerized and was permanently tacky at room temperature. Thecomposition exhibited cold flow, yet strips of it could be cut from abulk slab with a knife.

[0066] The prepared PSA composition was then fed via a Haake 18 mm twinscrew extruder into a 5 inch wide rotary rod die coater which appliedthe composition to the structured surface of the twelve body layers byhot melt coating. A rubber coated backup roll with a temperature varyingbetween about 90 and 180 degrees F (32 and 82 degrees C) was used duringthis application step. The composition temperature as applied wasdetermined by the extruder and die coater temperatures, ranging fromabout 325 to 375 degrees F (163 to 191 degrees C). Line speed of thebody layer during fill material application was between about 10 and 20ft/min (51 to 102 mm/sec). The applied composition coating wascontinuous and had a thickness of about 1-1.5 mils (25 to 38 μm)measured from the uppermost portions of the ridges on the structuredsurface. In all cases, voids between the composition and the reflectivefilm were observed at the cavity apices indicating poor replication-thecomposition had not fully filled the cube corner cavities (see FIG. 4A).The voids appeared to occupy approximately 10-40% of the cube cornercavity volume. The observed retroreflectivity was poor. For Examples 5-7and 11-13 the PSA composition was left as coated and for the othersamples the composition was crosslinked by exposure to a dosage of about440 mJ/cm² of UV light as measured with an EIT UVIMAP, NIST unitscalibrated in the UVA spectral range. The crosslinked composition formeda PSA with high shear and cohesive strength with little or no cold flow.Three different transparent cover layers, each about 2 mils (50 μm)thick were then laminated to the PSA composition layer of the samples atambient room temperature-an extruded impact-modifiedpolymethylmethacrylate (PMMA) film was laminated to Examples 5, 8, 11,and 14, a plastisol-coated plasticized polyvinyl chloride (PVC) film waslaminated to Examples 6, 9, 12, and 15, and an extruded polyethyleneco-acrylic acid (EAA) film was laminated to Examples 7, 10, 13, and 16.

[0067] The samples were wound up in rolls and maintained at ambient roomtemperature. Within several hours after lamination, the non-crosslinkedsamples (Examples 5-7 and 11-13) began to show significantly improvedretroreflective performance indicative of the fill material flowing tomore completely fill the cube corner cavities. After about 24 to 72hours the fill material layer for those samples was substantially freeof any voids. Apparently, the matter filling the voids diffused out ofthe sheeting during this time. The samples with the acrylic (PMMA) coverlayer appeared to allow the most rapid rate of self replication; thosewith the EAA cover layer were somewhat slower; and those with the vinyl(PVC) cover layer yielded the slowest self replication rates of thesamples tested. The crosslinked samples, however, showed no visibleimprovement in replication fidelity of the fill material layer evenafter several months of storage at ambient room temperature.

[0068] Samples similar to Examples 5, 7-11, and 13-16 were made exceptthat the body layer comprised polycarbonate (Makrolon brand, type 2407,available from Bayer) instead of polystyrene. The results followed thesame pattern as the polystyrene samples. Also, the effect of the use ornon-use of reduced gas pressure at the fill material application stationwas investigated; no effect was noted.

[0069] The non-crosslinked samples, after complete replication of thefill material into the cube corner cavities of the structured surface,were thus ready for subsequent exposure to radiation sufficient tocrosslink the fill material composition to increase its shear strengthby solidification, while still maintaining its PSA characteristics. Thecoefficient of retroreflection was measured for all the non-crosslinkedsamples for −4 degree entrance angle and 0.2 degree observation angle,with values ranging from about 589 to about 982 cd/lx/m².

EXAMPLE 17

[0070] A roll of body layer retroreflective sheeting substantially thesame as those of Examples 5-16 was prepared. The same structured surfacegeometry and reflective aluminum film was used.

[0071] A pressure-sensitive adhesive resin composition was prepared bymixing 75 parts of isooctyl acrylate and 25 parts of N-vinylcaprolactamto yield about 2000 grams. Then 0.05 pph of a photoinitiator(2,4,6-trimethylbenzoyldiphenylphosphine available as Lucirin™ TPO fromBASF Corp.) was added. Nitrogen was bubbled through the composition andthe composition was exposed to Sylvania black lights to partiallypolymerize it to a viscosity of about 1700 centipoise (1.7 Pa-s). Thecomposition viscosity was measured using a model LVF BrookfieldViscometer equipped with a number 4 spindle at 60 rpm at roomtemperature. In the partial polymerization process, the temperature ofthe composition increased from 23° C. to about 38° C. The compositionwas then sparged with air and cooled to room temperature. An additional0.15 pph of photoinitiator (Lucirin™ TPO) and 0.15 pph1,6-hexanedioldiacrylate were added to it. The partially polymerizedcomposition, still substantially monomeric (<10% polymerized), was knifecoated onto the body layer sheeting to a thickness of about 2 mils (50μm) measured from the uppermost portions of the ridges on the structuredsurface, and then exposed to ultraviolet radiation in a nitrogenatmosphere to cure (polymerize and crosslink) the adhesive in situ. Theultraviolet radiation was provided by ultraviolet black lamps havingmost of the light emission between 300 and 400 nanometers, and a peakemission at about 350 nanometers. The light intensity averaged about 4.9mW/cm², and the total energy was about 498 mJ/cm². The UV light wasmeasured with an EIT UVMAP in NIST units. Due to the low viscosity andthe wetting characteristics of the composition to the aluminum vaporcoat, no voids were seen in the fill material within seconds of coatingat the coating speed of 20 feet per minute. After curing, the adhesivewas covered with a silicone-coated polypropylene release liner.

[0072] Afterwards the release liner was removed and a transparent coverlayer of PMMA film similar to those used in Examples 5, 8, 11, and 14about 2 mils (50 μm) thick, was laminated to the pressure-sensitiveadhesive on the body layer to produce a sheeting construction. ThisExample 17 exhibited a good coefficient of retroreflectivity: theaverage value of measurements taken at 0 and 90 degree orientationangles, at a −4 degree entrance angle and 0.2 degree observation angle,was 1002 cd/lx/m².

Glossary of Selected Terms

[0073] “Adhesive” means a substance suitable for bonding two substratestogether by surface attachment.

[0074] The “body layer” of a retroreflective sheet or article that usesa structured surface for retroreflection is the layer (or layers)possessing the structured surface and chiefly responsible formaintaining the integrity of such structured surface.

[0075] “Cold flow” refers to the ability of a material to flow under itsown weight at ambient room temperature, about 20 degrees C.

[0076] “Cube corner cavity” means a cavity bounded at least in part bythree faces arranged as a cube corner element.

[0077] “Cube corner element” means a set of three faces that cooperateto retroreflect light or to otherwise direct light to a desiredlocation. “Cube corner element” also includes a set of three faces thatitself does not retroreflect light or otherwise direct light to adesired location, but that if copied (in either a positive or negativesense) in a suitable substrate forms a set of three faces that doesretroreflect light or otherwise direct light to a desired location.

[0078] “Cube corner pyramid” means a mass of material having at leastthree side faces arranged as a cube corner element.

[0079] “Cube height” or “cube depth” means, with respect to a cubecorner element formed on or formable on a substrate, the maximumseparation along an axis perpendicular to the substrate between portionsof the cube corner element.

[0080] “Diffusely reflective”, “diffuse reflectivity”, and cognatesthereof mean the property of reflecting a collimated incident light beaminto a plurality of reflected light beams. Surfaces that are diffuselyreflective also have a low specular reflectivity.

[0081] “Flowable” refers to the ability of a material to flow under itsown weight at a given temperature.

[0082] “Geometric structure” means a protrusion or cavity having aplurality of faces.

[0083] “Groove” means a cavity elongated along a groove axis and boundedat least in part by two opposed groove side surfaces.

[0084] “Groove side surface” means a surface or series of surfacescapable of being formed by drawing one or more cutting tools across asubstrate in a substantially continuous linear motion. Such motionincludes fly-cutting techniques where the cutting tool has a rotarymotion as it advances along a substantially linear path.

[0085] “Heat-activated adhesive” means a solid thermoplastic materialthat melts upon heating and then sets to a firm bond upon cooling.

[0086] “X% polymerized” means 100% minus the weight % of unreactedmonomer in a composition.

[0087] “Pressure-sensitive adhesive” (abbreviated “PSA”) means apermanently tacky material capable of adhering to surfaces upon applyingat least a slight amount of manual pressure.

[0088] “Radiation curable” means the capacity of a composition toundergo polymerization and/or crosslinking upon exposure to ultravioletradiation, visible radiation, electron beam radiation, or the like, orcombinations thereof, optionally with an appropriate catalyst orinitiator.

[0089] “Retroreflective” means having the characteristic that obliquelyincident incoming light is reflected in a direction antiparallel to theincident direction, or nearly so, such that an observer at or near thesource of light can detect the reflected light.

[0090] “Structured” when used in connection with a surface means asurface composed of a plurality of distinct faces arranged at variousorientations.

[0091] “Symmetry axis” when used in connection with a cube cornerelement refers to the axis that extends through the cube corner apex andforms an equal angle with the three faces of the cube corner element. Itis also sometimes referred to as the optical axis of the cube cornerelement.

[0092] “Transparent”, with regard to a layer or substance for use in aretroreflective sheeting, means able to transmit light of a desiredwavelength to a degree that does not prevent retroreflection.

[0093] “Top surfaces” of a structured surface that also containsrecessed faces refers to surfaces that are distinct from the recessedfaces and that have a minimum width in plan view of at least about0.0001 inches (2.5 μm).

[0094] “Viscosity” means the internal resistance to flow exhibited by afluid at a given temperature. Low to moderate viscosities are typicallymeasured using a rotating spindle in contact with the fluid andexpressed in SI units of Pascal-seconds (Pa-s). The viscosity of highlyviscous polymers can be measured by dissolving the polymer in a solventand comparing the efflux times, as described in Textbook of PolymerScience, F. W. Billmeyer, Wiley-Interscience (Second Edition, 1971), pp.84-85. A viscosity measured by the latter approach is referred to as“inherent viscosity” and expressed in units of inverse concentration,such as deciliters per gram (dl/g).

[0095] All patents and patent applications referred to herein areincorporated by reference. Although the present invention has beendescribed with reference to preferred embodiments, workers skilled inthe art will recognize that changes can be made in form and detailwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A retroreflective article, comprising: a bodylayer having a structured surface comprising recessed faces that definecube corner cavities; a reflective film disposed at least on therecessed faces; and a transparent adhesive layer that fills the cubecorner cavities.
 2. The article of claim 1 , wherein the adhesive layercomprises a transparent pressure-sensitive adhesive.
 3. The article ofclaim 1 , wherein the adhesive layer comprises a transparentheat-activated adhesive.
 4. The article of claim 2 , wherein thepressure-sensitive adhesive comprises a crosslinked polymer.
 5. Thearticle of claim 1 , wherein the adhesive layer is substantiallycoextensive with the structured surface.
 6. The article of claim 5 ,further comprising: a release liner that contacts the adhesive layer. 7.The article of claim 5 , further comprising: a transparent cover layerthat contacts and bonds to the adhesive layer.
 8. The article of claim 7, wherein the transparent cover layer comprises a thermoplastic polymer.9. The article of claim 7 , wherein the body layer comprises athermoplastic polymer.
 10. The article of claim 7 , wherein the adhesivelayer has a lower elastic modulus than that of the body layer.
 11. Thearticle of claim 10 , wherein the elastic modulus of the adhesive layeris less than about 345 MPa and the elastic modulus of the body layer isgreater than about 690 MPa.
 12. The article of claim 5 , wherein theadhesive layer is substantially continuous such that it covers both thecube corner cavities and upper portions of the structured surface. 13.The article of claim 1 , wherein the reflective film is discontinuous.14. The article of claim 1 , wherein the body layer also has a rearsurface opposite the structured surface, the article further comprising:a second pressure-sensitive adhesive layer disposed at the rear surface.15. A retroreflective article, comprising: a body layer having astructured surface comprising recessed faces that define cube cornercavities; a reflective film disposed at least on the recessed faces; anda layer of flowable radiation-curable composition that fills the cubecorner cavities.
 16. The article of claim 15 , wherein the compositionlayer is substantially coextensive with the structured surface.
 17. Thearticle of claim 16 , wherein the composition layer covers substantiallyall of the structured surface.
 18. The article of claim 15 , wherein thecomposition is substantially polymeric.
 19. The article of claim 15 ,wherein the composition is suitable for forming a transparentpressure-sensitive adhesive.
 20. The article of claim 15 , wherein thecomposition has a sufficiently low shrinkage such that upon curing itmaintains intimate contact with the recessed faces.
 21. The article ofclaim 15 , wherein the reflective film is discontinuous, and thecomposition is suitable for forming a covalent bond with exposedportions of the body layer.
 22. A method of making a cube cornerarticle, comprising: providing a body layer having a structured surfacethat includes recessed faces defining cube corner cavities; applying afilm of reflective material at least to the recessed faces; applying tothe structured surface a flowable composition suitable for forming atransparent pressure-sensitive adhesive; and exposing the composition toradiation sufficient to crosslink the composition after the compositionhas filled the cube corner cavities.
 23. The method of claim 22 ,further comprising: providing a first cover layer; and laminating thefirst cover layer to the article.
 24. The method of claim 23 , whereinthe second applying step applies the composition at a thicknesssufficient to form a composition layer covering the recessed faces andupper portions of the structured surface.
 25. The method of claim 24 ,wherein the first cover layer has the flowable composition appliedthereto, and the second applying step is carried out by the laminatingstep.
 26. The method of claim 23 , wherein the first cover layercomprises a release liner that does not bond to the composition.
 27. Themethod of claim 26 , further comprising: removing the release liner;providing a second cover layer suitable for bonding to the composition;and laminating the second cover layer to the composition.
 28. The methodof claim 22 , wherein the second applying step is carried out such thatthe flowable composition incompletely fills the cube corner cavities.29. The method of claim 28 , further comprising: providing a coverlayer; and laminating the cover layer to the article before the flowablecomposition has filled the cube corner cavities.
 30. The method of claim22 , wherein the flowable composition is at least 95% polymerized duringthe second applying step.
 31. A method of making a cube corner article,comprising: providing a body layer having a structured surface thatincludes recessed faces defining cube corner cavities; applying a filmof reflective material to the recessed faces; applying to the structuredsurface a radiation-curable composition suitable for bonding to the filmof reflective material; and exposing the composition to radiationsufficient to crosslink the composition after the composition has filledthe cube corner cavities.
 32. The method of claim 31 , wherein thecomposition is suitable for forming a transparent pressure-sensitiveadhesive.
 33. The method of claim 31 , further comprising: providing afirst cover layer; and laminating the first cover layer to thecomposition.
 34. The method of claim 31 , wherein the second applyingstep applies the composition at a thickness sufficient to form acomposition layer covering both the recessed faces and upper portions ofthe structured surface.